Some time ago I searched an application to tag files, eventually using multiple values separated by commas, optionally reusing tags saved in other files, and using a simple interface. (I had People in my mind)

Label can tag files and tint icons too, but it can't tag multiple values, so I did a YAB script that does it.

Tags used in other files could be selected only if there is a proper indexed attribute, like META:label ( but this can be modified to satisfy your tastes).

Thanks to xeD to made "catindex", a cli app that shows the values of an index, this make the script more quicker